Electrothermal control actuator



United States ELECTROTHERMAL CONTROL ACTUATOR Charles E. Markham, Aifton, Mo., assignor to Missouri Automatic Control Corporation, St. Louis, MO., a corporation of Missouri Application March 9, 1953, Serial No. 340,982

2 Claims. (CI. 60-43) This invention relates to electrothermal actuators and more paiticularly to the so called hot-wire type wherein a tensioned wire is heated by an electrical current so as to expand linearly to effect the actuation of a valve or switch or the like.

in these devices, there is the possibility of mechanical failure due to the hot wire element becoming overheated by an unforeseen rise in electrical current, and in many known uses to which the device is adaptable, such failure would result in a hazardous condition.

The primary object of the present invention is to provide a novel, fail-safe, hot-wire type actuator by which a valve or switch which is normally biased in its oil or safe position is moved to on position upon energization of the actuator and is permitted to return to its off or safe position in event the hot wire element breaks.

Another object is to provide a hot-wire type actuator in which spring means acts to move a control device from its normally biased off or safe position to its on position upon heating and expansion of the hot wire element, and in which this spring means is rendered ineffective in event of breakage of the hot wire element.

These and other objects and advantages which become apparent when reading the following description in connection with the accompanying drawing :are the purposes of the present invention.

The single figure of the drawing is a schematic illustration of a form of my invention adapted to operate a valve in a burner control system.

In the drawing, a valve body is indicated at having an inlet 11, an outlet 12, and a valve seat 13, a valve 14 mounted on a valve stem 15 cooperates with seat 13, and the stem 15 is guided in the upper part of the valve body. The upper end of valve stem 15 projects exteriorly of the valve body and the valve 14 is normally biased in a closed position by a spring 16.

Mounted on the valve body it? by means of screws 17 is a bracket 18 and attached to the upper end of bracket 13 by means of screws 19 is a vertical plate 20. The upper end of bracket 18 is provided with slotted holes 21 to permit vertical adjustment of the plate 20. The lower edge of plate is provided with a V-groove 22 which is engaged by the turned up, knife-edged end 23 of a lever 24. The other end of lever 24 overlays the upper end of the valve stem 15. A spring 25, having one end attached to the lever 24 at a point intermediate of its ends and having its other end anchored to the valve body, urges the lever 24 counterclockwise so as to push downward on valve stem 15 and open valve 14. Spring 25, it is to be understood, is considerably stronger than the valve closing spring 16.

Attached to the upper edge of plate 20, as by screws 26, is a cross piece 27 of dielectric material. A hot wire element 28, shown as an open end loop, is connected at its lower looped end to the lever 24 at a point thereon between pivot point 22 and the point of connection therewith of spring 25, and at its upper end to the cross piece 27 by passing them through perforations in the.

cross piece and welding them to perforated discs 31.

By way of illustration the device is shown as an element of a burner control system wherein it functions to operate the valve which controls the supply of fuel to the main burner in accordance with conditions of the control system. Other primary elements of the burner control system are; a space thermostat 32 which closes when burner operation is required to heat the space and a pilot burner safety switch 33 which closes and remains closed when the pilot burner is operating satisfactorily.

Thermostat 32 and safety switch 33 are included with the hot wire element 28 and a transformer 34 in a low voltage circuit which may be traced as follows: starting at one side of the transformer secondary winding 35, a lead 36, the space thermostat 32, a lead 37, the hot wire element 28, a lead 39, the pilot safety switch 33 and a lead 40 back to the other side of secondary winding 35. Power is supplied to the transformer primary winding 41 through leads 42 and 43 from an available source.

In operation, the hot wire element is energized to efiect opening of the main burner fuel supply valve upon demand for heat by the space thermostat, provided, of course, that the pilot burner switch 33 is closed. Upon energization, the hot wire element 28 is heated by the current passing through it and as a result it expands, permitting the left end of lever 24 to be moved down ward by the spring 25, thereby causing valve 14 to be opened. When sufficient heat has been supplied, space thermostat 32 opens breaking the circuit and hot wire element 28 cools and contracts thereby lifting the lever 24 against spring 25 and permitting valve 14 to close under the action of its return spring 16. The spring 25 constantly maintains the hot wire element in tension and constantly urges the lever 24 counterclockwise about its connection with the lower end of the hot wire element which, due to the resiliency of the element, acts as a pivot point, so that the knife edge 23 is constantly pressed in its groove 22. The valve return spring 16, being considerably weaker than spring 25, has no effect upon these conditions. Also, the permissible downward movement of the left end of lever 24 before it contacts the valve body is greater than any movement which could occur due to expansion of the hot wire element.

It, for any reason, the hot wire element 28 is broken, the lever 24 is immediately pulled downward away from its pivot point 22 and away from its contact with the upper end of valve stem 15, whereupon the valve 14 returns to its safe closed position under the action of spring 16. While the device is illustrated as a valve actuator, it is to be understood that it is equally adaptable to the operation of a switching device which is normally biased in a safe off position. It is to be further understood that the hot wire element 28 may be heated by a suitable resistance heating element in heat exchange relationship therewith instead of by the passing of a current therethrough, as shown, and that the hot wire element may consist of any number of strands.

The foregoing description is intended to be illustrative and not limiting, the scope of the invention being set forth in the appended claims.

I claim:

1. In a device of the class described, a substantially horizontal operating lever suspended at a point intermediate of its ends by a flexible, thermally expanding wire element, said lever having a free end arranged to laterally engage with the lower side thereof a device to be oper Patented Apr. 2, 1957 ated, an abutment at the other end of said lever and laterally engaged by the upper side of said lever, a spring acting downward on said lever at a point on the free end side of its point of connection with said element so as to cause saidlever tobe pivoted about its point of connection with said element and to constantly bear upwardly at its other end against said abutment, whereby as said element expands and contracts said lever pivots about its contact with said abutment and whereby in event said Wire element breaks said lever will be displaced from an operative position by said spring.

2. in a device of the class described, support structure, an operating lever, a movably mounted member, said lever being arranged to bear laterally against an abutment portion of said support structure and laterally against said movably mounted member at spaced points therealong, a flexible, thermally expandable wire element fixed at one end to said support structure and connected to said lever at its other end, the arrangement being such that said lever is held against said abutment by said Wire References Cited in the file of this patent UNITED STATES PATENTS 2,013,728 Fitz Gerald Sept. 10, 1935 2,203,719 Crane June 11, 1940 2,329,820 Breese Sept. 21, 1943 FOREIGN PATENTS 658,223 Germany Mar. 24, 1938 

